1. Field of the Invention
This invention relates to a speed controlling device for a rolling mill for minimizing a variation of a speed of rolls when a work to be rolled is introduced into the rolling mill.
2. Description of the Prior Art
Generally, a speed controlling device for a rolling mill is constructed as shown in FIG. 1. In particular, referring to FIG. 1, reference numeral 1 designates a work to be rolled, 2 a roll for rolling the work 1 to be rolled, 3 an electric motor for driving the rolls 2, 4 a rotational speed detector for detecting a rotational speed of the electric motor 3, 5 a reference speed signal, and 6 a speed controlling operational amplifier for receiving a signal representative of a rotational speed of the electric motor 3 from the rotational speed detector 4 to control the electric motor 3 to maintain a speed indicated by the reference speed signal. Reference numeral 7 denotes a current controlling device, 8 a current controlling signal developed from the operational amplifier 6, 9 a gate firing phase signal developed from the current controlling device 7, 10 a gate firing circuit, 11 a thyristor power supply unit for supplying power to the electric motor 3, and 12 a load current detector for detecting a load current flowing from the thyristor power supply unit 11 to the electric motor 3.
The control system of FIG. 1 can be represented in the form of a block diagram as illustrated in FIG. 2. Referring to FIG. 2, reference numeral 6' corresponds to the speed controlling operational amplifier 6 of FIG. 1, 7' to the current controlling device 7 of FIG. 1, and 8' to the current controlling signal 8. On the other hand, reference numeral 10' corresponds to the gate firing circuit 10 and the thyristor power supply unit 11 of FIG. 1, and a gain of a firing angle controlling section and a thyristor device of the same is represented by K. Further, reference numeral 3' corresponds to the electric motor 3 of FIG. 1.
Other reference numerals in FIG. 2 are listed below:
S; a Laplacean
K.sub.I ; a reset time of the current controlling section
K; a gain of the firing angle controlling section and the thyristor device
Ka; a constant regarding resistance of an armature
Ta; a time constant of an armature of the electric motor
Kc; a coefficient of induction
Km; moment of inertia of the electric motor and the rolls as a whole
T.sub.L ; load torque caused by rolling
Ks; a proportional gain of the speed controlling section
Ts; a proportional integration time constant of the speed controlling section
Referring again to the block diagram of FIG. 2, a transfer function from a speed N of the rolling mill to a joining point of outputs of the firing angle controlling section and the thyristor device both designated at 10' as a whole which presents the gain K, that is, a transfer function from the speed controlling operational amplifier 6 to the gate turning on circuit 10, is greater than the coefficient of induction Kc of the following stage, and hence a feedback loop providing the coefficient of induction Kc can be ignored. Further, a transfer function from an electric current I of the thyristor to a joining point of the current I and the current controlling signal 8' can be represented ##EQU1## provided T.sub.I can be approximated ##EQU2## where a break point frequency of a transfer function of the round is represented as .omega..sub.I.
Accordingly, a transfer function of a speed loop in the block diagram shown in FIG. 2 can be approximated as illustrated in FIG. 3. A Bode diagram in this case is illustrated in FIG. 4.
Generally in an apparatus such as a rolling mill, it is rotated at a fixed speed beforehand, and in this condition, a work to be rolled is introduced into the rolling mill, resulting in sudden application of a load to the rolling mill. Accordingly, where the rolling mill is under automatic speed control, it is commonly known that an electric current and a speed of an electric motor for driving the rolling mill vary as illustrated in FIG. 5. In particular, if a load is applied at a point of time 23, the speed 21 drops. Simultaneously, the current 22 increases to act to restore the speed 21 to an initial preset level. In this instance, an area 24 defined by a dropped amount .DELTA.N of the speed 21 and a time tr required for restoration of the same is used as a unit (index) which indicates a controlling performance of the rolling mill, and in view of characters of the rolling mill, it is considered desirable to minimize such an area 24. Variations of the speed 21 and the electric current 22 change depending upon a load to the electric motor.
In A and B of FIG. 5, an axis of abscissa represents lapse of time t, and an axis of ordinate in A of FIG. 5 indicates a speed N while an axis of ordinate in B of FIG. 5 indicates an electric current I. Reference numeral 21 denotes a variation of the speed, 22 a variation of the electric current, 23 a point of time at which a load is applied, and 24 an area on the drawing until the speed N which has varied returns to its initial level.
Thus, the area 24 defined by the variation .DELTA.N of the speed N and the restoration time tr where a rated load is applied is represented by an equation (1) below.
In particular, referring to FIG. 3, a transfer function G from load torque to a speed is ##EQU3##
However, since T.sub.I is significantly small when compared with 1/.omega..sub.c and can be ignored, let ##EQU4## then a following equation when the load torque is applied stepwise is obtained ##EQU5## (provided .phi.&gt;1).
Accordingly, the area 24 of FIG. 5 can be obtained by integration of the equation (2) from 0 to Tr. However, since t&gt;Tr and N.apprxeq.N, it can also be obtained by integration of a following equation (3) from 0 to .infin.: ##EQU6## where .omega.c=Ks/Km.
Hence, the area 24 can be reduced by making the .omega.c (or Ks) of the speed loop greater and reducing the time constant Ts of the same. However, values of these .omega.c and Ts are normally limited by DC current ripples caused by detection ripples of the rotational speed detector 4.
Ripples which are detected by the rotational speed detector 4 include ripples caused by the detector itself and ripples caused due to an error in centering which remains when the electric motor 3 and the speed detector 4 are connected directly to each other. Both kinds of ripples are produced at a rate of one period/one rotation, and especially ripples of the latter type are produced at a rate of one ripple/one rotation. Since this ripple value N.sub.R is amplified by means of the speed controlling operational amplifier 6, a ripple value Ks.multidot.N.sub.R appears in the electric current I.
In a rolling plant, normally a value of an electric current of an electric motor 3 for rolls during rolling is detected as torque generated in order to control the speed or the like of the entire rolling mill, and hence if current ripples increase, then a detection error becomes greater. Also during no load running, the speed is caused to vary by speed detection ripples, resulting in deterioration in preset accuracy before a work 1 to be rolled is introduced between the rolls 2. As a result, .omega.c cannot be raised so high, and if .omega.c is not raised high, then Ts cannot be reduced so small accordingly.
A speed controlling device for a rolling mill of such a conventional type as described above commonly employs a technique to interpose a correction signal in order to minimize the area 24 defined by the speed N and the restoration time tr of the rolling mill. For example, according to a technique disclosed in a Japanese Patent Publication No. 58-40437, a correction is made to a current controlling signal 8, and according to another technique disclosed in a Japanese Patent Application No. 51-124162, a correction signal is added to a reference speed signal. However, according to those techniques, setting of a constant in each of circuits for producing correction signals is delicate and must be necessarily adjusted while a work is being rolled. Thus, those techniques are disadvantageous in that a long time is required for such adjustment.